At least, your LTE provider is not forcibly redirecting DNS traffic to its own DNS servers (as flurry.com got resolved to 0.0.0.0 as expected).
But:
It would seem that unbound's full recursion isn't possible with your LTE connection.
Which ISP is providing your LTE connection?
I guess you did disable DNSSEC via Pi-hole's UI?
Note that unbound would still employ DNSSEC if you didn't adjust its configuration files.
With DNSSEC enabled, you could try to see where DNSSEC validation fails.
Retrieve and store the DNS root servers DNSKEYs:
dig . DNSKEY | grep -Ev '^($|;)' > root.dnskeys
Then use those DNSKEYs to validate a domain lookup, e.g.:
dig +sigchase +trusted-key=./root.dnskeys heise.de
If that doesn't reveal anything (trouble-shooting SERVFAILs is an arduous process), you could further analyse your issue, e.g. by trying deHakkelaar's suggestions following my remark in SERVFAIL with deutsche-glasfaser.de - #12 by Bucking_Horn.